Chapter 6
Conclusions and Future Work
6.1 Conclusions
Complex FBG structures of arbitrary phase shifts and refractive index profiles have been continuously attractive for many optical applications. To develop the state-of-the-art FBG fabrication technique, in this dissertation we have proposed and demonstrated several advanced FBG fabrication methods for achieving pure apodization, long grating length with sequential UV writing, and better linear index response.
This dissertation firstly presents a new sequential UV-writing procedure for fabricating long fiber Bragg grating (FBG) devices in Chapter 3. In the literature, several procedures that can realize long and complex FBG structures have recently been developed. However, the accumulative position reading errors have caused significant difficulties on the fabrication of long-length fiber Bragg gratings. Based on the side-diffraction method, we have proposed and demonstrated a real-time fiber position monitoring method for sequential UV-writing processes, in order to be able to write long-length gratings accurately. To real-time accurately align the position of every exposed FBG section prior to UV exposure, a single-period reference fiber grating with strong refractive index modulation is probed by applying an interferometric side-diffraction method to measure the grating phase as the position reference. In this way the overlapped FBG sections can be connected section-by-section without obvious phase errors, even when the written
69
index-modulation is weak.
To realize FBGs with arbitrary refractive index profiles and arbitrary phase shifts, it is necessary to calibrate the relationship between UV beam exposure duration versus refractive index change. In Chapter 4, we elucidate the refractive index change response of the photosensitive optical fiber to exposed UV flux. The nonlinear UV photosensitivity deforms the index profile of FBGs on the tail edges where the writing UV flux is low. Pre UV treatment eliminates the nonlinear index-change response and ensures the linearity of the response of subsequent grating writing. A new method that involves interference between two unequal beams in a single writing scan is proposed and demonstrated to be able to improve the index profile and the spectral response of fabricated FBGs. The procedure is stable for writing weak fiber Bragg gratings with pre-designed index profiles.
In Chpater 5, a simple UV exposure method is proposed to achieve pure apodization for fiber Bragg gratings fabricated by sequential UV writing. Through the exposure phase and/or time control of multiple UV shots, the ac-index can be adjusted independently with the dc-index kept constant. The UV dose exposed on the fiber to form every grating section is divided into two sequentially writing shots instead of one.
In this way one gains the freedom to adjust the ac-index independently while keeping the dc index profile fixed. By precisely connecting grating sections with partial overlap, the desire grating profile can be matched while the dc index become constant through the whole grating length. The proposed simple and cost-effective FBG writing method is able to realize pure apodization for both the phase mask and holographic sequential UV writing schemes
Based on the developed grating inscription skills and calibration results in this dissertation, we have developed an excellent fiber grating fabrication platform which
70
has the potential to be able to produce complex and advanced FBGs for various applications.
6.2 Future Work
This dissertation demonstrates improved FBG fabrication methods which are proven to be feasible. Future work on FBG development along this line are listed in the following sections, including complex FBG fabrication, FBG sensor applications, and FBGs in fiber lasers.
6.2.1 Complex FBG Fabrication
Since our goal is to develop a plateform to fabricate various kinds of FBGs, the next work is to actually realize a more complicated designed grating structure. In the past years, our group have tried some numerical simulation methods to inversely design the detail parameters of FBG structures from a given spectrum [6.1,6.2]. The FBG fabrication can be carried out in principle by the proposed techniques in Chapter 3, 4, and 5. However, smaller UV writing beam size is required to be able to fit the severely changed grating index profiles. Take Ref. [6.2] for example, the multi-channel FBG structure is very difficult to realize by using the ordinary partially-overlapped sequential writing process with the Gaussian-shaped UV fringe with its 1/e2 width about 6.5 mm. For this case, we will use smaller UV writing beam size to fit the target index profile, and sequentially write the profile into the fiber by using the side-diffraction method for real-time monitoring and by the new pure apodization method. Furthermore, the reference fiber grating used in Chapter 3 makes the optical alignment difficult because of the small fiber radius and the weak diffraction efficiency.
71
The idea of replacing the reference fiber grating with a phase mask as a position reference is considered. Though the phase mask has the period twice of the written grating, the reliable accuracy of the period and the stable properties makes the phase mask a good candidate as the reference for position monitoring. The work is already going on right now.
6.2.2 FBG Sensor Applications
Fiber Bragg grating-based sensing has proven to be a very fertile research area, and new developments can be expected to continue over the next few years. Gratings are simple, intrinsic sensing elements which can be photo-inscribed into a silica fiber.
Grating-based sensors appear to be useful for a variety of applications, including refractive index, temperature, and tension sensing. Our next work is trying to apply FBGs and LPGs into environmental sensing.
6.2.3 FBGs in Fiber Lasers
Many research works have been focused on the development of 1 μ m mode-locked Yb-fiber lasers recently, since 1μm femtosecond pulsed source are useful in many applications, such as multiphoton microscopy [6.3], supercontinuum generation for optical coherent tomography (OCT) [6.4], and high-repetition-rate femtosecond combs for precise frequency metrology [6.5]. Our group had achieved 1.55μm femtosecond Er-fiber lasers, and can also apply to 1μm femtosecond Yb-fiber lasers. FBGs written in Yb-doped photosensitive fiber to directly form a DBR fiber laser is our next project.
72
6.3 References
[6.1] L.-G. Sheu, K.-P. Chuang, and Y. Lai, “Fiber Bragg grating dispersion compensator by single-period overlap-step-scan exposure,” IEEE Photon.
Technol. Lett. 15, 939-941 (2003).
[6.2] C.-L. Lee, R.-K. Lee, and Y.-M. Kao, “Design of multichannel DWDM fiber Bragg grating filters by Lagrange multiplier constrained optimization,” Opt.
Express 14, 11002-11011 (2006).
[6.3] T.-H. Tsai, S.-P. Tai, W.-J. Lee, H.-Y. Huang, Y.-H. Liao, and C.-K. Sun,
“Optical signal degradation study in fixed human skin using confocal microscopy andhigher-harmonic optical microscopy,” Opt. Express 14, 749 (2006).
[6.4] H. Lim, Y. Jiang, Y. Wang, Y.-C. Huang, Z. Chen, and F. W.
Wise,“Ultrahigh-resolution optical coherence tomography with a fiber laser source at 1μm,” Opt. Lett. 30, 1171 (2003).
[6.5] T. Udem, R. Holzwarth, and T. W. Hänsch, “Optical frequency metrology,”
Nature 416, 233 (2002).
73